Note that there should not be any module declaration in an Erlang
script file. Instead, the first line is usually the interpreter line
which invokes escript. If you invoke escript like this

$ escript factorial 5

the contents of the first line does not matter, but it cannot contain
Erlang code as it will be ignored.

The second line in the example, contains an optional
directive to the Emacs editor which causes it to
enter the major mode for editing Erlang source files. If the
directive is present it must be located on the second
line.

On the third line (or second line depending on the presence
of the Emacs directive), it is possible to give arguments to
the emulator, such as

%%! -smp enable -sname factorial -mnesia debug verbose

Such an argument line must start with %%! and the
rest of the line will interpreted as arguments to the emulator.

If you know the location of the escript executable, the first
line can directly give the path to escript. For instance:

#!/usr/local/bin/escript

As any other kind of scripts, Erlang scripts will not work on
Unix platforms if the execution bit for the script file is not set.
(Use chmod +x script-name to turn on the execution bit.)

An Erlang script file must always contain the function main/1.
When the script is run, the main/1 will be called with a list
of strings representing the arguments given to the script (not changed or
interpreted in any way).

Call escript:script_name/0 from your to script to retrieve
the pathname of the script (the pathname is usually, but not always,
absolute).

By default, the script will be interpreted. You can force it to be
compiled by including the following line somewhere in the script file:

-mode(compile).

It is not necessary to export the main/1
function when the script is interpreted or dynamically
compiled.

It is also possible to have a precompiled script which
contains a beam file. In a precompiled script, the
interpretation of the script header is exactly the same as
in a script containing source code. That means that you can
make a beam file executable by prepending the file
with the lines starting with #! and %%!
mentioned above. In a precompiled script, the function
main/1 must be exported.

As yet another option it is possible to have an entire
Erlang archive in the script. In a archive script, the
interpretation of the script header is exactly the same as
in a script containing source code. That means that you can
make an archive file executable by prepending the file with
the lines starting with #! and %%! mentioned
above. In an archive script, the function main/1 must
be exported. By default the main/1 function in
the module with the same name as the basename of the
escript file will be invoked. This behavior can be
overridden by setting the flag -escript main Module
as one of the emulator flags. The Module must be the
name of a module which has an exported main/1
function. See code(3)
for more information about archives and code loading.

Warning

The support for loading of code from archive files
is experimental. The sole purpose of releasing it before it
is ready is to obtain early feedback. The file format,
semantics, interfaces etc. may be changed in a future
release. The flag -escript is also experimental.

Pre-processor directives in the script files are ignored,
with the exception for the -include_lib
directive. For instance, use

-include_lib("kernel/include/file.hrl").

to include the record definitions for the records used by the
file:read_file_info/1 function.

Pre-defined macros (such as ?MODULE) will not work.
A script does not have module name, so BIFs such as
spawn/3
that require a module name cannot be used.
Instead, use a BIF that take a fun, such as
spawn/1.

The script will be checked for syntactic and semantic correctness
before being run. If there are warnings (such as unused variables),
they will be printed and the script will still be run. If there are
errors, they will be printed and the script will not be run and its
exit status will be 127.

If the main/1 function in the script returns successfully,
the exit status for the script will be 0. If an exception is generated
during execution, a short message will be printed and the script terminated
with exit status 127.

Options accepted by escript

-s

Only perform a syntactic and semantic check of the script file.
Warnings and errors (if any) are written to the standard output, but
the script will not be run. The exit status will be 0 if there were
no errors, and 127 otherwise.